I am fairly sure those memory chips won't work in an X58 mobo. They are designed for xeon systems. 12GB is really the max available at the moment, given there are not freely available 4GB DDR3 consumer RAM DIMMS.

Saying that, 12GB shouldn't hurt your performance at all. What may improve it is upgrading from 4850X2s to 4870X2s or better still GTX295s.

The hard drives could also be a lil faster. Either 4 x velociraptors or if you're super keen go for your favourite hardcore SSD, pref. something intellish.

My 2c, personally not the system I would build, but if you are looking for ultimate bang for back, they would be clear enough upgrades and considering the money you would save on the RAM and going for an i7 920 instead of 965EE, you wouldn't be out of pocket by much.

Cheers

PS: Quick peek at MSI.com shows that (Samsung K4B2G0846B-HCF8) was the only 4GB DIMM actually tested to run with that MB you mentioned. Good news is it worked fine with 6 DIMMS. Not sure if the crucial DIMMs use samsung or hynix chips though...